The aim of this study was to evaluate the inclusion of two macroalgae as dietary ingredients on growth, body composition, intermediate metabolism, intestinal enzymes and gut histology of Sparus aurata juveniles. Six isocaloric and isonitrogenous diets incorporating 5%, 15% or 25% of Gracilaria cornea or Ulva rigida meal were tested against a control diet. Duplicate groups of fifteen fish (14 g) were fed the experimental diets for 70 days. Growth performance showed an inverse linear relationship with the inclusion level of Gracilaria. Feed conversion ratio increased significantly in fish fed 25% Gracilaria. Ulva presented a quadratic relationship with the lowest SGR at 15% of inclusion. Fish fed 25% Ulva exhibited significantly lower lipid content than those fed control diet. Overall, metabolites did not show differences with specimens fed control diet, except for plasmatic glucose and muscle lactate. Intestinal proteolytic activity was reduced with the macroalgae inclusion level, but all fish showed the same enzyme pattern. The histological study revealed no negative effect on liver and intestine structure by the macroalgae inclusion. The present work demonstrated that these ingredients can be used in practical diets for sea bream, but a more comprehensive research is needed to determine the optimum dietary level in a longer feeding trial.
Seaweeds growing in the intertidal zone are exposed to fluctuating nitrate and ultraviolet radiation (UVR) levels. While it has been shown that elevated UVR levels and the decrease of nitrate concentration can reduce photosynthetic levels in seaweeds, less is known about the combined effect of nitrate levels and UVR on metabolism and photoprotection mechanisms of intertidal species. Consequently, the objective of this study was to evaluate the effect of nitrate concentration and UVR treatments on photosynthesis, respiration, nitrate reductase activity and phenolic compound levels of Ulva rigida (Chlorophyta). There was a two-to threefold increase in maximal gross photosynthesis (GP max ) and respiration rates, as nitrate increased from 0 to 50 μM NO 3 − . Similarly, nitrate reductase activity increased linearly from low values in algae incubated at 0 μM NO 3 to high values in tissue incubated at 50 μM NO 3 − . Phenolic compounds in the tissue of U. rigida increased approximately 60% under 50 μM NO 3 − relative to those incubated at 0 μM NO 3 − . Algae exposed to UVR (8 h) showed a significant decrease in the effective quantum yield and respiration, however, no effect was observed in the phenolic compounds levels. Full recovery of effective quantum yield was observed after U. rigida was transferred for 48 h to low PAR. Nitrate reductase also decreased after an 8-h UVR exposure, but no differences were observed among the nitrate treatments. This study shows that high nitrate levels reduced the negative effect of UVR on the effective quantum yield and increased the recovery of key metabolic enzymes. It is possible that the increase of phenolic compounds in the thallus of U. rigida under high nitrate levels provide a photoprotective mechanism when exposed to high UV levels during low tides.
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